Industrial gas turbines generally include a compressor, a combustion system and a turbine. The combustion system commonly includes a plurality of combustors disposed at circumferentially spaced intervals in a ring about the periphery of the gas turbine. In each combustor, liquid or gas fuel is combusted in high pressure air delivered by the compressor to produce high temperature, high pressure combustion gases. A transition duct connects the outlet end of each combustor to the inlet end of the turbine whereby the high temperature, high pressure combustion gases formed in the various combustors are delivered to the turbine. The turbine is driven in rotation as the high temperature, high pressure combustion gases expand in passing over the turbine blades. The turbine drives the compressor and, in power plant applications, also drives a generator to produce electricity.
Combustors used in industrial gas turbines of this type typically include multiple fuel nozzles, generally five or six arrayed in a circular ring pattern. In some embodiments, an additional fuel nozzle is disposed centrally within the ring of fuel nozzles and along a central axis of the combustor. The combustor includes a generally cylindrical outer casing extending longitudinally about a central axis from a forward end to an aft end in gas flow communication with the turbine. A generally cylindrical combustor liner, which circumscribes and defines the combustion chamber, is disposed coaxially within a generally cylindrical flow sleeve that is coaxially disposed within the outer casing of the combustor. A combustor liner cap assembly is disposed coaxially at the forward end of the combustor liner to form the closed forward end of the combustor liner and is fixed to the outer casing of the combustor.
The combustor liner cap assembly conventionally includes a generally cylindrical sleeve secured to a base plate and extending longitudinally to an aft plate disposed at the aft end of the combustor liner. The forward plate is a generally cylindrical plate having a plurality of nozzle openings, one for each fuel nozzle assembly. An open-ended cylindrical premix tube extends aftward from each of the nozzle openings. Additionally, a plurality of collar-like nozzle guides, one per nozzle opening, are disposed about the nozzle openings of the face of the base plate.
The construction of the combustor liner cap assembly must allow for differential thermal expansion at the downstream end of premix tubes and at the same time provide a structurally sound attachment of the premix tubes to the combustor structure. Further, the nozzle guides must be retained in place against the face of the base plate in a “floating” relationship that permits slight adjustment relative to the nozzle openings to accommodate insertion of a fuel nozzle assembly through the nozzle guide and into the associated premix tube. The conventional construction of the combustor liner cap assembly could be improved to be more cost-effective and less labor intensive, while maintaining, if not improving, the construction of the combustor liner cap assembly with respect to the afore-mentioned criteria.